Large adaptive secondary mirrors are a promising solution for the next generation of high order adaptive optics systems and are under development in all major observatories. One of the key points of these systems is the manufacturing of the large glass thin shell, often convex aspheric, used as the 'mirror surface' and attached to the actuators. Due to the very tight surface quality specifications for high order or extreme adaptive optics systems (XAO), one of the major challenges is to avoid all high spatial frequencies errors during the manufacturing of these extremely thin convex hyperbolic shells. In order to achieve the required surface quality, we present an active optics technique based on elasticity theory and mirror polishing under constraints, allowing to easily generate highly aspheric optics, using only full size tools (spherical or flat), and avoiding such high spatial frequencies defects. The proposed original process for thinning, smoothing and polishing a large (1.1m) thin (2mm) shell has been modelled using finite elements method. The feasibility of this process is demonstrated. Results in terms of load optimization, evolution of stresses and constraints within the shell and expected surface optical quality are presented.